The Collaborative International Dictionary
Isogeotherm
Isogeotherm \I`so*ge"o*therm\, n. [Iso- + Gr. gh^ the earth + qe`rmh heat.] (Phys. Geog.) A line or curved surface passing beneath the earth's surface through points having the same mean temperature.
Wiktionary
isogeotherm
n. A geoisotherm.